On Saturday Zimbabweans will go to the polls to approve a new constitution that has been many years in the making, but which some feel has not been worth the effort. The new constitution will be the last major hurdle before the country can hold elections, probably in late July.

That vote will once again pit Robert Mugabe, now 89, and Morgan Tsvangirai against each other. Mugabe shows no signs of having lost the taste for power, and Tsvangirai has certainly shown that he appreciates its privileges. A splendid mansion, shopping trips abroad, and amorous adventures seem evidence of that. Even so, Tsvangirai is the only candidate who proposes a new start, and the west has invested heavily in him. If he wins, the country may indeed get its new start but if he loses the kind of compromise coalition that South Africa's Thabo Mbeki negotiated last time around may be impossible.

The new constitution has no place for a prime minister – Tsvangirai's current position – and the role of vice presidents is not suitable for powerful or even influential personalities with demanding parties at their heels.

The words of the UN secretary-general, urging that last week's Kenyan elections should be "credible and peaceful" have set a tone. No one is talking "free and fair". If Zimbabwe's referendum on Saturday is credible and peaceful, the UN, the Commonwealth, the southern African region and many others will breathe a sigh of relief.

The constitution, which is almost certain to be approved, is a curious document. In some ways it resembles closely a draft that Mbeki's negotiators put forward half a dozen years ago, the so-called Kariba draft, as it was put to the Zimbabwean parties on a houseboat moored on Lake Kariba. In others, it recognises some of the positive aspects of the new Kenyan constitution and makes a faint-hearted but discernible effort to enshrine citizen rights. The chief complaint, one made consistently by civil society groups, is that the president retains too much power.

Such power might well be required by a victorious Tsvangirai to unite the country and take it forwards, but the unspoken fear is that it will be a possible licence for abuse by a triumphant Mugabe – who may be within reach of a "credible and peaceful" victory which would hardly be free and fair.

Having said that, the powers of the president in the new constitution are not greater than those granted in France. If the president dies in office, then the right of the senior vice president to succeed and hold power without election is just as it is in the United States.

But this is where the constitution is a strange beast. It is clearly a document with last-minute compromises embedded within it – including one that states that if the president dies or stands down in the first 10 years after its adoption, the succession would pass to a nominee of the president's own party. This is seen as a safeguard for Mugabe's Zanu-PF party but, even with this exceptional provision, it is not likely Tsvangirai would be made senior vice president in a coalition government dominated by Mugabe.

Behind the scenes, all parties are talking about compromises, coalitions, immunities from prosecution. The problem is that everyone wants to dominate any coalition. And that's because no one is really certain who might win. The most comprehensive polls, although with huge margins of error, seem to favour Mugabe – even in a free and fair contest. But that huge margin of error could work in many ways. Certainly it could disguise a huge disillusionment within the electorate and there may well be massive abstention or spoiling of ballots. With this uncertainty about the margin of error, the fear is that Zanu-PF will intimidate and rig again.

The referendum this weekend will attract a small vote. The weariness in the country accepts that a compromise constitution is the best available. Whether the electoral commission can conduct the referendum smoothly – the country still has difficulty funding democratic exercises – is the burning question. No one wants the March referendum to prefigure a July election which is botched administratively rather than won or lost peacefully, credibly, freely or fairly. In the meantime, the Law Society of Zimbabwe is right to say that there is something to celebrate in the constitution. It provides an expansive Bill of Rights with citizen capacities to enforce those rights in law. Gender rights are very visible. Powers are clearly separated to protect the courts. Such a constitution, with a government that observed it in good faith, would be workable and a massive improvement. But the question in today's Zimbabwe is precisely to do with good faith.